(1) Field of the Invention
The present invention relates to a method for welding conductive terminals each made of copper to each other by resistance welding, and a conductive terminal structure obtained by the method.
(2) Description of the Related Art
Typically, a resistance welding method has been used for welding conductive terminals in an electronic device to each other. An arc welding method and other welding methods are used rarely due to the following reason. That is, an amount of electric current to be fed in welding is too large; therefore, there is a possibility that an electronic component is damaged.
In resistance welding, when electric current is fed to components to be welded each of which is resistant to electricity in a state where the components come into contact with each other, the components are melted by heat generated owing to the electrical resistance.
As one example of the resistance welding method, there has been known a spot welding method (see FIG. 4 in JP2002-095134A and FIG. 2 in JP2002-281644A). This method involves a step of butting flat-shaped conductive terminals with each other, a step of bringing the conductive terminals into contact under pressure with electrodes each having a spherical or convex tip end such that the conductive terminals are sandwiched between the electrodes, and a step of feeding electric current to the electrodes. According to the spot welding method, a spot-shaped nugget (a melting portion) is formed at a contact portion between the conductive terminals. As another example of the resistance welding method, there has been also known a projection welding method (see FIG. 3 in JP2002-095134A and FIG. 1 in JP05(1993)-283139A). This method involves a step of butting a first conductive terminal including a spherical or convex projection against a second conductive terminal, a step of bringing the conductive terminals into contact under pressure with flat-shaped electrodes such that the conductive terminals are sandwiched between the electrodes, and a step of feeding electric current to the electrodes. According to the projection welding method, the projection itself forms a nugget.
According to the former spot welding method, conductive terminals each made of iron or the like can be readily welded to each other. According to the latter projection welding method, a projection itself forms a nugget; therefore, a melting state of a weld portion can be relatively stabilized.
In the spot welding method, however, conductive terminals each made of copper with considerably small resistivity, in particular, oxygen free copper with high purity are hardly welded to each other for the following reason. That is, copper effects less heat generation owing to electrical resistance and less melting by the heat; therefore, welding becomes difficult. Even when the surface of the conductive terminal made of copper is subjected to tin plating in order to ensure electrical resistance to some extent, it is difficult to form a nugget stably. As shown in FIG. 3 in JP2002-095134A and FIG. 1 in JP05(1993)-283139A, in the conventional projection welding method, a center of a tip end portion of a conductive terminal is protruded to form a spherical or convex projection. Consequently, it is considerably difficult to enhance an accuracy of an amount of the projection which is melted into an opposite conductive terminal to form a nugget. Hence, the formation of the nugget becomes unstable and, also, a strength (a welding strength) of the nugget becomes unstable. As shown in FIG. 3 in JP2002-095134A, further, the projection is formed in such a manner that the center of the tip end portion of the conductive terminal is protruded. Therefore, it is difficult to spread an area of the projection. This causes problems that an adequate strength can not be ensured and an electric current capacity of the nugget is small. In the projection welding method, similarly, conductive terminals each made of copper with considerably small resistivity are hardly welded to each other for a reason that copper effects less heat generation owing to electrical resistance and less melting by the heat. Therefore, the welding of the conductive terminals becomes considerably difficult.